SQL Server 2017

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create table #Dataset 
(
    ID int not null,
    LName varchar(50) null,
    Fname varchar(50) null,
    DateOfBirth varchar(50) null,
    StreetAddress varchar(50) null,
    Source varchar(50) null,
)

insert into #Dataset (ID, LName, Fname, DateOfBirth, StreetAddress, Source)
values
('1', 'John', 'Ganske', '37171', '  1223 Sunrise St', 'Company XYZ'),
('2', 'John', 'Ganske', '37171', '  1233 Sunrise St', 'Company XYZ'),
('4', 'Brent', 'Paine', '20723', '  5443 Fox Dr', Null),
('3', 'Brent', 'Paine', '20723', '  5443 Fox Dr', 'Company XYZ'),
('5', 'Adam', 'Smith', '22805', '  1254 Lake Ridge Ct', Null),
('6', 'Adam', 'Smith', '22805', '  1254 Lake Ridge Ct', Null),
('7', 'Adam', 'Smith', '22805', '  1254 Lake Ridge Ct', 'Company XYZ'),
('8', 'Timothy', 'Johnson', '36165', '  1278 Lee H-W', Null),
('9', 'Timothy', 'Johnson', '36165', '  1278 Lee H-W', Null),
('10', 'Judy', 'Wilson', '32579', '  5678 Dotties Dr', 'Company XYZ'),
('12', 'Peter', 'Pan', '37507', NULL, Null),
('11', 'Peter', 'Pan', '37507', NULL, 'Company XYZ');

--select * from #Dataset
WITH cte AS (
SELECT *, 
  SUM(CASE WHEN Source IS NULL THEN 1 ELSE 0 END) OVER(PARTITION BY LName, Fname, DateOfBirth, StreetAddress) c1,
  SUM(CASE WHEN Source = 'Company XYZ' THEN 1 ELSE 0 END) OVER(PARTITION BY LName, Fname, DateOfBirth, StreetAddress) AS c2,
  COUNT(*) OVER(PARTITION BY LName, Fname, DateOfBirth, StreetAddress) c3
FROM #Dataset
)
SELECT *
FROM cte 
WHERE c1 > 0
  AND c2 > 0
  AND c3 > 1
  AND Source IS NULL
ORDER BY ID

ID	LName	Fname	DateOfBirth	StreetAddress	Source	c1	c2	c3
4	Brent	Paine	20723	5443 Fox Dr	null	1	1	2
5	Adam	Smith	22805	1254 Lake Ridge Ct	null	2	1	3
6	Adam	Smith	22805	1254 Lake Ridge Ct	null	2	1	3
12	Peter	Pan	37507	null	null	1	1	2